MIPS64: Fix NoBarrierAtomicValue.Construction test failure on big-endian

Reason for the failure is that the test enumeration is 32-bit wide, whereas
AtomicWord is 64-bit wide on 64-bit machines. On 64-big endian, this loads the random four bytes located after the 32-bit value that is tested.

BUG=
TEST=unittests/NoBarrierAtomicValue.Construction

Review-Url: https://codereview.chromium.org/2464703003
Cr-Commit-Position: refs/heads/master@{#40767}

test/unittests/base/atomic-utils-unittest.cc

@@ -84,7 +84,7 @@ TEST(AtomicNumber, OperatorSubtractionAssignment) {
 
 namespace {
 
-enum TestFlag {
+enum TestFlag : base::AtomicWord {
   kA,
   kB,
   kC,